The present invention relates to a hydrometallurgical process for the treatment of a raw material which contains iron and other metals, by means of a sulfuric acid solution in order to dissolve the metals and to precipitate the iron.
Various processes for the treatment of zinc ferrite have been used in conjunction with the electrolytic zinc process. The present invention is not, however, limited to the treatment of zinc ferrites, but it can be generally applied to the treatment of zinc-, copper-, cobalt-, nickel-, cadmium-, magnesium-, and manganese-bearing raw materials which contain iron and which, when dissolved in sulfuric acid, form Fe.sub.2 (SO.sub.4).sub.3 - and MeSO.sub.4 - bearing solutions (Me=Zn, Cu, Co, Ni, Cd, Mg, Mn). Zinc ferrite (ZnFe.sub.2 O.sub.4) is produced in the roasting of iron-bearing sulfidic acid concentrates. It is the most important zinc-bearing secondary component in the calcine. The zinc bound in it can amount to ten percent, or more, of the entire zinc content of the calcine. The principal component of the calcine is zinc oxide. The calcine is leached in a sulfuric acid-bearing solution. The leaching process yields a virtually iron-free (iron concentration&lt;20 mg/l) "neutral" zinc sulfate solution. This is achieved in the so-called neutral leach, in which the initial pH range is 1.5-2 and the final pH range is 4-5. Most of the zinc oxide of the calcine dissolves under these conditions. The iron which has possibly passed into the solution early during the leach precipitates towards the end of this stage as hydroxide. After this stage the solution is fed to solution purification and then to electrolysis, in which the zinc is precipitated as metal and the sulfuric acid is regenerated. This regenerated solution, the return acid, is returned to the calcine leach, and thereby a closed solution cycle is obtained. A closed solution cycle has many advantages, but in practice difficulties are also encountered. One difficulty is the insufficient volume of water used for washing the precipitates which have been removed from the process, as the maintenance of the water balance in the leach process does not tolerate the admission of very large washing water volumes into the system. Another difficulty involved in a closed solution cycle is the concentration of certain elements, especially magnesium, in the cycled electrolyte; their removal from the electrolyte requires considerable additional measures and expense.
The leach residue from the neutral stage consists mainly of zinc ferrite, which does not dissolve under the conditions of this leaching stage. For a long time ferrite residue constituted a problem in zinc processing, since suitable hydrometallurgical methods had not been developed for its treatment. The principal problem was the lack of a technologically implementable method for the precipitation of iron. After 1965, however, experts in the field became generally aware of a zinc process in which the iron was precipitated as a crystalline jarosite compound (AFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 ; A=Na, NH.sub.4) which can be easily separated from the solution. In this process the ferrites are leached in a sulfuric acid solution, a normal return acid, whereby a sulfuric acid-bearing solution of iron and zinc sulfates is obtained. The leaching stage is normally called strong acid leach. The average concentrations of the solutions produced in it are as follows: [H.sub.2 SO.sub.4 ].apprxeq.50 g/l, [Fe.sup.3+ ].apprxeq.35 g/l, and [Zn.sup.2+ ].apprxeq.100 g/l. The pH of the free sulfuric acid is raised, using calcine, to approximately 1.5 and the iron is precipitated in the presence of NH.sub.4.sup.+, Na.sup.+ or K.sup.+ ions by maintaining the pH at the said value by means of additions of calcine as jarosite according to Reaction Equation (1). EQU 3Fe.sub.2 (SO.sub.4).sub.3 (aq)+6ZnO(s)+Na.sub.2 SO.sub.4 (aq)+6H.sub.2 O(aq).fwdarw.2Na[Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 ](s)+6ZnSO.sub.4 (aq). (1)
By this precipitation process a zinc sulfate solution is obtained which has a relatively low iron concentration; the solution is fed directly to the neutral leach. The calcine quantity required by the neutralization of acid and the precipitation of jarosite is, in general, considerable. In most cases it is approximately 30% of the total feed of calcine. The ferrites present in this calcine do not dissolve under the iron precipitation conditions and remain in the jarosite precipitate. A process has been developed for the recovery of the zinc present in the ferrite, i.e. an acid wash of the jarosite precipitate, in which the ferrites present in the precipitate are dissolved, while the jarosite remains undissolved (Norwegian Pat. No. 123,248). In this zinc ferrite process, an additional stage, pre-neutralization, is often used between the strong-acid leach and the jarosite precipitation. During this preneutralization stage, free acid is neutralized up to the jarosite precipitation point by means of calcine; this intermediate stage diminishes the need for calcine in jarosite precipitation. It can be seen that the neutralization required by iron precipitation, as well as the feed of calcine to the precipitation stage, complicates the process to a considerable degree. Besides, the insoluble components of calcine remaining in the jarosite precipitate render the precipitate so impure that it is not a suitable raw material for iron production processes. Furthermore, the lead, silver and gold contents of the calcine fed into the precipitation process are also lost; in some cases these contents have a value high enough to make their recovery economically worthwhile. In general it is necessary to drive the jarosite precipitate, with its lead, silver and gold contents, to a waste disposal area. Owing to the limited washing-water intake capacity which is characteristic of the process, the washing of the jarosite precipitate remains insufficient. For this reason the precipitate further contains considerable quantities of heavy metals (Zn, Cd, Cu) in a water-soluble form. This involves a considerable loss of the metal. In addition, water-soluble heavy metals constitute an environmental hazard and must be brought, by a suitable treatment of the precipitate, to scarcely soluble forms, harmless for the environment.
Subsequent processes have been developed which eliminate some of the drawbacks of the process described above. There is, for example, a process in which the dissolution of the ferrites and the precipitation of jarosite take place simultaneously (U.S. Pat. No. 3,959,437), in which case a high recovery of zinc, copper and cadmium is achieved by simple apparatus and processes. Nevertheless, even in this case, the lead, silver and gold contents of the calcine are lost, along with the jarosite precipitate, and the iron precipitate is not a suitable raw material for iron production.
In another process (Japanese Pat. No. 48-7961) the iron is precipitated in an autoclave. The objective is to obtain an iron precipitate so pure that it is suitable as such for iron production. In this process the ferrites are leached by means of sulfuric acid and SO.sub.2 gas. The iron present in the solution is now in the ferrous form. The solution is neutralized before the precipitation of iron. The iron is oxidized to the ferric form and is precipitated in an autoclave. The aim of this process is to precipitate iron as haematite (Fe.sub.2 O.sub.3). However, the formation and stability conditions of haematite set certain limitations on the precipitation and thereby also on the process. The precipitation temperature being between 180.degree. and 200.degree. C., Fe.sub.2 O.sub.3 is not stable above a sulfuric acid concentration of 60-65 g/l. When ferrous iron is precipitated as haematite, sulfuric acid is released at 1.75 g H.sub.2 SO.sub.4 /g Fe; when ferric iron is precipitated as haematite, sulfuric acid is released at 2.6 g H.sub.2 SO.sub.4 /g Fe. In the former case--assuming that the free acid is completely neutralized--iron can be precipitated as haematite at a maximum rate of 33-36 g Fe/l and in the latter case at approximately 23-24 g Fe/l without autoclave neutralization of the acid produced in the reaction. If a solution with relatively high iron and sulfuric acid concentrations (e.g. [Fe.sup.3+ ].apprxeq.50-100 g/l and [H.sub.2 SO.sub.4 ].apprxeq.50-100 g/l) is fed into the autoclave, the iron precipitates, when the temperature in the autoclave is approximately 180.degree.-200.degree. C., as a basic sulfate, FeSO.sub.4 OH (E. Posnjak, H. E. Merwin, J. Amer. Chem. Soc. 44 (1922) 1965). It proves impossible to combine simple precipitation of pure haematite in an autoclave and the aim, natural in terms of simplification of the process, of diminishing the volume flows of the process solutions, which would lower investment and heating costs, but which would also increase the iron and acid concentrations in the solutions.
In all the zinc ferrite treatment processes described, considerable quantities of neutralizing agent are required. Normally, in zinc plants, neutralizing agent is available in sufficient quantities in the form of zinc oxide present in the zinc calcine. Attempts at solving this problem have usually resulted in multi-stage processes, losses of valuable metals, and large quantities of polluting wastes.
There are also cases in which neutralizing agents are not available in sufficient quantities. This is the case especially in the treatment processes of various complex ores. For example, many sulfidic Zn-Pb, Zn-Cu, Cu-Pb, and Zn-Cu-Pb concentrates may contain such large quantities of iron that, when they are subjected to an oxidizing roasting at approximately 900.degree. C., the product of the roasting may contain primarily ferritic components and only a lesser quantity of pure oxide phases. Processes of the types described above cannot be used for the treatment of such roasting products, since sufficient quantities of natural neutralizing agent, the pure oxide phase of the calcines, are not available for the neutralization of the acid solutions produced by the leach of the ferrites and by the precipitation of iron.
The problem of treating ferritic raw materials of the above type is, however, solved by the process according to the present invention. The invention is also highly applicable to the treatment of ferrites in connection with zinc processes, and it solves several problems involved in the electrolytic zinc process.